OK after a little more reading I have come to the concusion that I was talking bollocks.
Oh BTW you are also wrong T-B, a paintball marker can no way be compared to a real gun.
I think the optimal length is 12-14". This gives the ball enought length to accelerate (6-8") and the rest will give it a little stability.
Anything over this will cause excess friction on the ball reducing velocity and accuracy. A well machined barrel combined with correctly sized paint will give better results than a longer, inferior barrel. So the ideal barrel would be 12-14" long with porting after 6-8" of its length.
Again feel free to tell me I am talking ****. I normally do